Lecture 03 Variables Prof. Katherine Gibson Prof. Jeremy Dixon - - PowerPoint PPT Presentation

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CMSC201 Computer Science I for Majors

Lecture 03 – Variables

• Prof. Katherine Gibson
• Prof. Jeremy Dixon

Based on slides by Shawn Lupoli and Max Morawski at UMBC

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Last Class We Covered

• Algorithms
• Program Development
• Control Structures

– Sequential – Decision Making – Loops

• Types of Errors

– Syntax – Logic

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Any Questions from Last Time?

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Exercise

• What will each of the following do?

1.print("Hello') Error – Need to have matching ' and " 2.Print('Hello') Error – Need to have lowercase print 3.print('Hello World') Hello World

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Today’s Objectives

• To start learning Python
• To learn more about variables

– How to use them – Different types

• To learn how to use input and output

– To do interesting things with our program

• To play a party game

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“Cowboy Coding”

• Jumping right in to writing code
• Disadvantages

–No formal management of project –No standard way of coding –Not planning things out

• Forgetting to include important things
• Having to make big changes later

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Software Development Process

• 1. Analyze the problem

– Determine specifications (requirements)

• 2. Create a design
• 3. Implement the design
• 4. Test and debug the program
• 5. Maintain the program

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Example: Temperature Converter

You have been invited to live in Europe during a semester abroad. You aren’t sure how to dress because the temperature is given in Celsius.

• Problem:

–Temperature is given in Celsius

• Solution:

–Write a program to convert Celsius to Fahrenheit

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Input/Process/Output

• Input

– What information do you need for your converter?

• Process

– What formulas do you need for your converter?

• Output

– What is the output from your converter?

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Introduction to Python (Variables)

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Python

• Python is a widely used language

– General purpose – High-level language

• Emphasizes code readability

– More streamlined than some other languages

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“Hello World!”

• In Python:

print("Hello World!")

• In the C++ programming language:

#include <iostream> int main() { std::cout << "Hello, world!\n"; }

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Elements of a Program

• Identifiers

– Variables – Modules (later in the semester) – Functions (later in the semester)

• Expressions

– Code that manipulates or evaluates identifiers

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We Start Python Today!

• Two ways to use python

–You can write a program as a series of instructions in a file and then execute it –You can also test simple Python commands in the Python interpreter

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We will write programs

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What Is a Variable?

• Something that holds a value

– Can change (unlimited number of times)

• Similar to variables in math
• In simple terms, a variable is a

“box” that you can put stuff in

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Rules for Naming Variables

• Variables can contain:

– Uppercase letters (A-Z) – Lowercase letters (a-z) – Numbers (0-9) – Underscores (_)

• Variables can’t contain:

– Special characters like $, #, &, ^, ), (, @

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More Rules for Naming Variables

• Variables can be any length

– x – IsKanyeRunningForPresidentIn2020 – myName

• Variables cannot start with a digit

– 2cool4school is not a valid variable – cool4school is a valid variable

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Variables and Keywords

• Keywords are the reserved words in Python
• Variables cannot be keywords

– or is not a valid variable name – orange is an acceptable variable name

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False class finally is return None continue for lambda try True def from nonlocal while and del global not with as elif if

• r

yield assert else import pass break except in raise

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Exercise: Variables

• Are the following legal or illegal in Python?

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1spam

No – Illegal!

raise1

Yes – legal!

Spam_And_Eggs

Yes – legal! But it doesn’t follow

• ur coding standards!

spamAndEggs or spam_and_eggs

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Using Variables in Python

• Create a variable by declaring it
• Also need to initialize it

– Use the assignment operator (=)

richFiddy = 50000000 poorFiddy = 0.50 brokeFiddy = 0

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assignment operator

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Introduction to Python (Expressions)

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Expressions

• Programs manipulate data

–Allows us to do interesting things

• Expressions calculate new data values
• Use assignment operator to set new value

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Expressions Example

numCandy = 10 priceCandy = 0.50 totalCandy = numCandy * priceCandy

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assignment operator variable being set value (a “literal”) expression

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Common Mistake

• Many new programmers mix up the left and

right hand sides of the assignment operator

• Variable being set is on the left
• Expression is on the right

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numCandy = 10 10 = numCandy 



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Variable Types

• There are many different kinds of variables!

–Numbers

• Integers
• Floats (decimals)

–Booleans (True and False) –Strings (collections of characters)

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Variables Types: Examples

aString = "Hello class" float_1 = 1.12 myBool = True anInteger = 7 dogName = "Mrs. Wuffington" classCode = 201

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Variable Usage

• Variables are designed for storing information
• Any piece of information your program uses
• r records must be stored in a variable

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Introduction to Python (Input and Output)

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Output

• Output is text printed to the screen

–So the user can see it and respond

• The command for this is print

– Use the keyword “print” and put what you want to be displayed in parentheses after it

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Output Example

print (3 + 4) print (3, 4, 3 + 4) print() print("The answer is", 3 + 4) 7 3 4 7 The answer is 7

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Output Exercise 1

• What will the following code snippet print?

a = 10 b = a * 5 c = "Your result is: " print(c, b) Your result is: 50

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Output Exercise 2

• What will the following code snippet print?

a = 10 b = a a = 3 print(b) 10

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There are two possible

• ptions for what this

could do! Any guesses?

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Output Exercise 2 Explanation

• Why does it print out 10?
• When you set one variable equal to another,

they don’t become linked!

• After b is set to 10, it no longer has

anything else to do with a

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Output Exercise 2 Explanation

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a = 10 b = a a = 3 print(b)

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a b

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Output Exercise 2 Explanation

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a = 10 b = a a = 3 print(b)

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a

10

b

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Output Exercise 2 Explanation

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a = 10 b = a a = 3 print(b)

3

a

10

b

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Output Exercise 2 Explanation

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a = 10 b = a a = 3 print(b)

3

a

10

b

• utput: 10

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Input

• Input is text we get from the user

– We must tell them what we want first

userNum = input("Please enter a number: ") print(userNum)

• The output will look like this:

Please enter a number: 10

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How Input Works

userNum = input("Please enter a number: ")

• Takes the text the user entered and stores it

– In the variable named userNum

• You can do this as many times as you like!

userNum = input("Enter another number: ") userNum2 = input("Enter a new number: ") userAge = input("Please enter your age: ")

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Input as a String

• Everything that comes through input()

will come in the form of a string

• There is a difference between "10" and 10

– "10" is a two character long string – 10 is understood by Python as a number

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Converting from String

• To turn an input string into a number,

you can do the following:

aNum = input("Enter a number: ") aNum = int(aNum)

• int stands for integer (a whole number)
• You can also do it in one line:

aNum = int(input("Enter a number: "))

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Class Exercise: Mad Libs

• Mad Libs is a phrasal template word game

where one player prompts others for a list

• f words to substitute for blanks in a story,

before reading the – often comical or nonsensical – story aloud

• The game is frequently played as a

party game or as a pastime

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Exercise: Calculating Averages

• Write, on paper or on your computer, a

program that asks the user for two numbers and prints out the average.

• Make sure to use variables, and to get the

input from the user!

• Does the order of operations come into play

for this exercise?

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Exercise: Assignment Weighting

• Pretend you’re writing a program to compute

someone’s weight grade. You have so far: hwWeight = 0.4 examWeight = 0.5 discussionWeight = 0.1

• Write a program that then asks the user for their

homework grade, exam grade, and discussion grade and prints out their total grade in the class.

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Announcements

• Your Lab 0 is in class this week!

– Go to your scheduled location and time

• Homework 1 is out (on Blackboard)

– Due by next Monday (Feb 8th) at 8:59:59 PM – You must have completed the Syllabus Quiz to see it

• Academic Integrity Quiz on Blackboard

– Must complete to see Homework 2 next week

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Practice Problem

• You recently accepted a new job in Philadelphia, PA

and you are trying to figure out if you should take the train or drive.

– Taking the train costs $4.90 each way – It is 75 miles round trip, your car averages 28 mpg, and gas is $2.03 per gallon plus $0.06 per mile in maintenance. Your new work has free parking, so you don’t need to pay.

• Write a program that can calculate the total cost to

drive to work, and print out if it is cheaper to drive to work, or to take the train. By how much?

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